In the design of various analog and digital circuits, such as D/A converters, buffer arrangements, and voltage regulators, it is necessary to establish a stable bias reference within the circuit. The bias reference will ideally be substantially independent of variation in both temperature and power supply. Either currents or voltages may serve as references, but voltages are often preferred in order to facilitate interface with the remainder of the circuit.
Reference voltage circuits can be classified by the source of the voltage standard from which, for example, a bias current may be derived. Convenient standards include the base-emitter voltage of a bipolar transistor, the thermal voltage, V.sub.T, and the breakdown voltage of a Zener diode. Each of these can be utilized in order to establish power-supply independence, but the first two alternatives are relatively sensitive to temperature variation. Although the reference voltage produced by Zener diodes is generally less dependent on temperature, variation on the order of +200 to +500 ppm/.degree.C. can be expected. Accordingly, some form of compensation must be employed in order to meet typical integrated circuit (IC) temperature stability requirements of .ltoreq.50 ppm/.degree.C.
Conventional temperature compensation schemes utilized in monolithic IC design generally rely on some type of component matching techniques to reduce reference variation. In particular, a predictable source of temperature drift is paired with another source of opposite polarity which can be scaled by a temperature-independent scale factor. Then, through appropriate circuit design, the effects of the two opposite-polarity drifts are made to cancel. Such an approach, however, requires an initial prediction as to the manner in which each source drifts with temperature. Moreover, the reliability of such a forecast diminishes when the temperature coefficients of the matched components are relatively large or nonlinear. It follows that a compensation network capable of being adjusted subsequent to physical realization would be of significant utility.
As is well known, variation in IC fabrication processes also tend to cause the magnitude of voltage references to deviate from a desired value. Such process variation can reduce manufacturing yields when an anticipated value of a voltage or current is altered as a consequence of reference voltage variation. As a specific example, processing-induced reduction in the "trip current" of sense amplifiers used in conjunction with memory circuits often renders them unusable given the precision with which this current must be controlled. A considerable yield reduction results since the trip current may not be increased subsequent to device fabrication.
Accordingly, a need in the art exists for a voltage generator for producing a reference voltage capable of being adjusted subsequent to device fabrication.